1. Field of the Invention
The present invention generally relates to spinning parachutes and, more particularly, is concerned with an annular rotating flexible decelerator type spinning parachute for decelerating and transmitting torque to a scanning submunition enabling it to search a spiral path in a target area.
2. Description of the Prior Art
A wide area mine (WAM) submunition is currently being developed for the U.S. military. The WAM (also termed a "smart" mine) submunition emplaced on the ground has a noise sensor that can detect the sounds, or acoustic signature, of a heavy vehicle, such as a tank, in a target area around its periphery.
When a tank is detected in the target area, the WAM submunition launches a sublet from its launcher tube on a ballistic trajectory over the target area. Then, a spinning parachute is deployed which slows the sublet and places it in an inward spiraling descent over the target area with a scanner in the sublet scanning the area beneath the descending sublet until the target is located. Once the sublet has a fix on the target, a warhead detonates, sending an explosively-formed projectile into the target at a high velocity to achieve destruction of the target.
As shown in FIG. 1, a spinning type parachute A is typically used for decelerating and transmitting torque to the scanning sublet B for enabling it to search a spiral path C in a target area. The efficiency of this search is improved by decreasing the distance between subsequent scans, called the lacing distance R. For a fixed hang angle, the lacing distance R will decrease when the parachute descends slower and/or spins faster. Thus, the higher the ratio of spin rate p to vertical descent velocity V of the spinning parachute, the more efficient the search pattern.
Existing spinning parachute designs are: (1) a round parachute modified to spin by cutting exhaust vents in the panels from which air can be deflected to induce a spin; (2) a cross or cruciform parachute modified to spin by shortening the suspension line(s) on one edge of each panel to create a pitch angle; (3) a vortex ring parachute which has four panels, each held at a pitch angle by a series of various length suspension lines along its perimeter; and (4) a rotating flexible decelerator parachute being a flat circular round parachute with one edge of each gore attached to one radial line and the opposite edge flying free which when inflated causes the gores to assume a pitch angle, thereby inducing a spin, and provides an exhaust vent along the entire radius.
Spinning parachute designs are judged on the basis of performance, weight, volume and producibility. Parachute performance, in general, is represented by drag, stability and deployability. In addition, spinning parachute performance is represented by spin rate and advance ratio, or the ratio of spin rate to descent velocity. Drag and payload weight determine descent velocity with drag coefficient (C.sub.d) based on the fabric area, fabric permeability and parachute type.
Stability refers to degree of oscillation. A spinning sublet requires a stable platform (with minimal oscillation) for aim accuracy. Stability is important because large oscillations can cause unpredictable descent velocities and flight paths. Stability improves as porosity is increased (i.e. more porous fabric and/or larger vent areas); however, increased porosity has the negative effect of decreasing drag.
Deployability or parachute inflation results from a pressure differential between the inside and outside of the folded parachute because of a higher freestream velocity along the outside surface. If parachute solidity (fabric area/disk area) is too low, the freestream velocity through the inside of the folded parachute will be too high to induce a pressure differential sufficient to induce inflation.
As with any flight system, weight of the parachute should be minimized. Low volume is also particularly important for packing efficiency of the parachute. Therefore, it is desirable to choose a spinning parachute design that uses as little material and as simple a structure as possible.
Finally, parachute producibility is dependent upon structural complexity and strength requirements. In most instances, parachutes do not lend themselves well to mass production. The elasticity of parachute materials, particularly that of the fabric which due to the variability related to the weave orientation, can cause problems with deformation during the sewing process, thereby causing problems with dimensioning and tolerancing. Tolerancing errors increase cost and reduce reliability. Structural simplicity contributes to producibility, which relates directly to cost.
A desirable goal is to design a spinning parachute that maximizes drag, spin rate and stability, while minimizing weight and volume for applications such as the WAM sublet noted above. None of the above-mentioned existing spinning parachute designs, except the rotating flexible decelerator parachute satisfy the need for such applications. Consequently, the invention described herein satisfies a need for certain types of payloads that require a rotating decelerator.